I am trying to understand how the SSIM and VMAF filters work, with an
eye to finding the "best" compression settings for a video which will
be composed from a series of TIFF images. Unfortunately, I'm stuck at
the beginning, as I can't get the SSIM filter to behave as expected.
./source contains the original sequence of images.
$ tiffinfo source/000000.tif
TIFF Directory at offset 0x473108 (4665608)
Image Width: 1440 Image Length: 1080
Bits/Sample: 8
Sample Format: unsigned integer
Compression Scheme: None
Photometric Interpretation: RGB color
Samples/Pixel: 3
Rows/Strip: 1
Planar Configuration: single image plane
I attempt to create a lossless video of the first minute.
$ ffmpeg -start_number 0 -framerate 60000/1001 -i source/%06d.tif -t
00:01:00 -c:v huffyuv lossless.mkv
The result appears reasonable.
$ mediainfo lossless.mkv
General
Unique ID :
235140899628261703308032414639716345340 (0xB0E67D6EF6B78362D9BCF9EA3080A5FC)
Complete name : lossless.mkv
Format : Matroska
Format version : Version 4
File size : 8.54 GiB
Duration : 59 s 994 ms
Overall bit rate : 1 223 Mb/s
Writing application : Lavf58.45.100
Writing library : Lavf58.45.100
ErrorDetectionType : Per level 1
Video
ID : 1
Format : HuffYUV
Format version : Version 2
Codec ID : V_MS/VFW/FOURCC / HFYU
Duration : 59 s 994 ms
Bit rate : 1 199 Mb/s
Width : 1 440 pixels
Height : 1 080 pixels
Display aspect ratio : 4:3
Frame rate mode : Constant
Frame rate : 59.940 FPS
Color space : RGB
Bit depth : 8 bits
Scan type : Progressive
Bits/(Pixel*Frame) : 12.860
Stream size : 8.37 GiB (98%)
Writing library : Lavc58.91.100 huffyuv
Default : Yes
Forced : No
Now let's see what the SSIM filter says.
$ ffmpeg -i lossless.mkv -start_number 0 -framerate 60000/1001 -i
source/%06d.tif -t 00:01:00 -filter_complex ssim -f null -
...
Input #0, matroska,webm, from 'lossless.mkv':
Metadata:
ENCODER : Lavf58.45.100
Duration: 00:00:59.99, start: 0.000000, bitrate: 1223104 kb/s
Stream #0:0: Video: huffyuv (HFYU / 0x55594648), bgr0, 1440x1080,
59.94 fps, 59.94 tbr, 1k tbn, 1k tbc (default)
Metadata:
ENCODER : Lavc58.91.100 huffyuv
DURATION : 00:00:59.994000000
Input #1, image2, from 'source/%06d.tif':
Duration: 01:47:16.00, start: 0.000000, bitrate: N/A
Stream #1:0: Video: tiff, rgb24, 1440x1080, 59.94 tbr, 59.94 tbn,
59.94 tbc
...
[Parsed_ssim_0 @ 0x55fb09c738c0] not matching timebases found between
first input: 1/1000 and second input 1001/60000, results may be incorrect!
...
[Parsed_ssim_0 @ 0x55fb09c738c0] SSIM R:0.833774 (7.793009) G:0.835401
(7.835723) B:0.831058 (7.722615) All:0.833411 (7.783532)
That's not what I expected. My understanding is that the R, G, B, and
All values should all be "1.000000 (inf)".
The "not matching timebases" warning is the obvious thing to look at.
After much searching, I came upon the -video_track_timescale option, but
it seems to only take an integer, and 60 is not the same as 59.94, so it
seems that I simply can't directly compare a video stream with a non-
integer framerate to an image sequence.
As a workaround, I tried extracting the lossless video frames as a
separate image sequence.
$ ffmpeg -i lossless.mkv -start_number 0 lossless/%06d.png
This created the expected sequence of image files (000000.png -
003595.png). Since I have both the "source" and the "lossless" streams
as images sequences, I can use ImageMagick to compare them.
$ for I in `seq -w 0 3595` ; do compare -metric AE source/00${I}.tif
lossless/00${I}.png /tmp/diff.png 2>/dev/null || echo $I ; done
This produces no output, indicating that ImageMagick thinks that the
TIFF files in ./source and the PNG files in ./lossless contain
completely identical image data. What does the SSIM filter say?
$ ffmpeg -framerate 60000/1001 -start_number 0 -i lossless/%06d.png
-framerate 60000/1001 -start_number 0 -i source/%06d.tif -t 00:01:00
-filter_complex ssim -f null -
...
Input #0, image2, from 'lossless/%06d.png':
Duration: 00:00:59.99, start: 0.000000, bitrate: N/A
Stream #0:0: Video: png, rgb24(pc), 1440x1080, 59.94 fps, 59.94
tbr, 59.94 tbn, 59.94 tbc
Input #1, image2, from 'source/%06d.tif':
Duration: 01:47:16.00, start: 0.000000, bitrate: N/A
Stream #1:0: Video: tiff, rgb24, 1440x1080, 59.94 tbr, 59.94 tbn,
59.94 tbc
...
[Parsed_ssim_0 @ 0x556c7e948980] SSIM R:0.999775 (36.484195)
G:0.999777 (36.508857) B:0.999774 (36.451744) All:0.999775 (36.481536)
Close, but not the "1.000000 (inf)" that I would expect for identical
streams. I suppose that it may be related to the "rgb24(pc)" vs.
rgb24 pixel formats; it's hard to be sure, as Google isn't turning up
anything that describes what "rgb24(pc)" actually means.
Does anyone know what the heck is going on here? Am I doing something
wrong?
--
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In Soviet Russia, Google searches you!
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